A patch clamp capacitance recording system has been set up with a resolution of 0.1 fF, allowing high resolution measurements of secretion in the patch. We have applied this system to RBL cells, a cell line commonly used for studying secretion. While we have seen endocytosis (downward steps in capacitance), no exocytotic steps have been observed during secretion. Instead, we see spikes in the capacitance, possibly indicating transient fusion events where the vesicle fuses only briefly with the plasma membrane. Work is underway to confirm the nature of the capacitance spikes and explore their dependence on external calcium and other conditions necessary for secretion